The present invention relates to an image display unit and a production method thereof, and particularly to a technique of producing an image display unit including display elements such as LED pellets arrayed in a matrix pattern.
FIG. 7A is a schematic perspective view showing one example of a related art image display unit used for flat TV sets. Wiring portions 8 and extraction electrodes 8a for external connection are formed on the surface of an alumina board 7. The surface of the alumina board 7, excluding the wiring portions 8 and the extraction electrodes 8a, is covered with a black resin. Part of each wiring portion 8 is taken as a pad electrode on which an LED (Light Emitting Diode) pellet is fixed. Pad electrodes are formed on top surfaces of the LEDs and are electrically connected to the extraction electrodes 8a via fine wires 20.
FIG. 7B is a schematic partial sectional view of the image display unit shown in FIG. 7A. As shown in this figure, the LED pellets are connected, by die bonding, to the wiring portions 8 on the alumina board 7. The surface of the alumina board 7, excluding the wiring portions 8, is covered with the black resin 23. Each LED pellet has a P-N junction layer. The pad electrode 22 is formed on the top surface of the LED pellet. The pad electrodes 22 are electrically connected to the extraction electrodes 8a by wire bonding using the fine wires 20. When a current is supplied to each LED pellet with the pad electrode 22 side taken as the positive side and the wiring portion 8 side taken as the negative side, light is emitted from the P-N junction layer of the LED pellet, to obtain a desired image display.
Image display units using LEDs (Light Emitting Diodes) as display elements are classified into outdoor type units and indoor type units. In particular, indoor type units have a COB (Chip On Board) structure as shown in FIGS. 7A and 7B, in which semiconductor chips are directly mounted on a board. Image display units, each having a pixel pitch ranging from 2.5 mm to 8 mm, have been commercially available. A semiconductor chip (LED pellet) has a size of about 0.3 mm×0.3 mm. As a result, in case of producing a color image display unit having an array configuration that red semiconductor chips, green semiconductor chips, and blue semiconductor chips are arrayed in a delta pattern, the pixel pitch exceeds 1 mm.
On the other hand, a ratio of the cost required for producing semiconductor chips to the total cost required for producing an image display unit is relatively high. Accordingly, in comparison of a small-sized display unit with a large-sized display unit, if the total number of pixels of the small-sized display unit is equal to that of the pixels of the large-sized display unit, the production cost of the small-sized display unit is not different from the production cost of the large-sized display unit so much. This means that the small-sized display unit is comparatively expensive in consideration of its screen size smaller than that of the large-sized display unit. Accordingly, the demand to reduce the production cost of the small-sized display unit is stronger than the demand to reduce the production cost of the large-sized display unit.
By the way, the pixel pitch can be reduced by reducing the size of each semiconductor chip. This leads to a possibility of a reduction in production cost of a micro-display unit. The reduction in size of each semiconductor chip, however, has a problem that the handling thereof becomes difficult and thereby the production cost is raised. For example, in case of handling a semiconductor chip with a vacuum chuck, the size of the vacuum chuck must be reduced along with the reduction in size of the chip, with a result that it is difficult to prepare a handling jig, to raise the production cost.
Various method of rationalizing the handling of LED pellets have been proposed. One method, disclosed in Japanese Patent Laid-open No. Sho 56-17385, is shown in FIGS. 8A to 8E.
Referring first to FIG. 8A, an LED wafer 1, on which a p-n junction layer and electrodes (not shown) are previously formed, is stuck on a temporary board 4 made from polyvinyl chloride to which an acrylic adhesive is previously applied.
Referring to FIG. 8B, the wafer 1 is separated into individual LED pellets by a dicing machine using a diamond blade. Since the dicing machine can cut the wafer on an accurate cycle, the individually separated LED pellets are kept in a state being orderly arrayed on the temporary board 4.
Referring to FIG. 8C, the LED pellets are transferred onto a second temporary board 6. The second temporary board 6 is also formed of a sticky sheet made from polyvinyl chloride to which an acrylic adhesive is previously applied, and accordingly, the LED pellets can be easily transferred onto the second temporary board 6. Subsequently, a conductive paste 30 is applied to selected LED pellets, for example, by screen printing.
Referring to FIG. 8D, the temporary board 6, on which the LED pellets are left stuck, is overlapped to a mounting board 7 made from ceramic or glass fiber reinforced epoxy resin on which die pads 8 are previously formed, whereby the LED pellets facing to the die pads 8 are transferred from the temporary board 6 to the mounting board 7.
Referring to FIG. 8E, the LED pellets thus transferred on the mounting board 7 are fixed thereto by the conductive paste 30. In accordance with such a method, fine LED pellets can be orderly arrayed at specific intervals on the mounting board 7.
In case of producing a high definition color image display unit, the pixel pitch is required to be reduced to a value equal to or less than 1 mm, and in this case, the size of each semiconductor chip must be reduced to a value equal to or less than 0.1 mm. It is difficult to handle such fine chips by the above-described related art method shown in FIGS. 8A to 8E. The related art method characterized by selectively applying a conductive paste on LEDs by screen printing has, if the size of pellets to be handled become fine, a limitation in terms of printing accuracy and also has a limitation in terms of accurate handling because of the use of the conductive paste which is in a fluid state.